Method of operating an analyzer



E. c. MILLER 3,015,985

METHOD oF OPERATING AN ANALYZER '3 Sheets-Sheet 1 Jan. 9, 1962 originalFiled Aug. 2o, 195e E. c. MILLER 3,015,985

METHOD oF OPERATING AN ANALYZER 3 Shees-Sheewl 2 Jan. 9, 1962 OriginalFiled Aug. 20, 1956 INVENTOR. E C MILLER A TTORNEVS llllnll mg wo. wwe.

United States Patent O METHOD F OPERATING AN ANALYZER Elmer C. Miller,Bartlesville, Okla., assignor to Phillips Petroleum Company, acorporation of Delaware Original application Aug. 20, 1956, Ser. No.604,991, now Patent No. 2,890,571, dated June 16, 1959. Divided and thisapplication Nov. 12, 1958, Ser. No. 773,371

2 Claims. (Cl. 88-14) This invention relates to a differentialrefractometer which is particularly adapted for use in analyzing gaseousstreams. In another aspect it relates to apparatus for equalizing tluidpressures between two chambers.

I'his application is a division of my copending application Serial No.604,991, filed August 20, 1956, now Patent 2,890,571 issued .lune 16,1959.

It is known that ethylene and other low molecular weight unsaturatedhydrocarbons can he produced advantageously by the thermal cracking oflight saturated hydrocarbons, such as butane. The effluent from such acracking process comprises vapors which have boiling points over a widetemperature range. One convenient system for separating desired gases,such as ethylene, involves initially removing the C4 and heavierhydrocarbons in an absorption column. The eiiiuent from this column canbe fractionated to remove methane and other components which haveboiling points lower than the boiling point of methane. The kettleproduct from this fractionation column can further be fractionated toseparate the gaseous mixture into streams of selected constituents.

In accordance with my copending application Serial No. 604,991, there isprovided a method for separating a uid mixture containing methane andother components having higher boiling points into a iirst stream whichis substantially free of methane and a second stream which containsmethane and other components in the huid mixture which have boilingpoints lower than the boiling point of methane. 'This separation isaccomplished by the use of a fractionation column. The overhead streamremoved from the column comprises primar-ily methane and otherconstituents having lower boiling points. The kettle product issubstantially free of methane. It has been discovered that therefractive index of the uid mixture in the lower region of the column isindicative of the methane concentration in the mixture. This issurprising because the material in the lower region of the columnnormally contains a large number of constituents other than methane. Therefractive index of a sample stream removed from the lower region of thecolumn is measured to provide a signal which is representative of themethane concentration at this region of the column. This signal isemployed to control the operation -of the column to maintain the methaneconcentration in the kettle product within selected limits. The controlcan be accomplished advantageously by regulating the amount of overheadgases removed from the system.

The refractive index measurement is advantageously made by comparing therefractive index of the sample stream in the gaseous state with therefractive index of a reference gas. It is desirable to maintain thesample stream at a relatively high pressure in order to obtain maximumsensitivity. In accordance with the present invention, a system isprovided whereby the pressures of the two gases being compared can bemaintained constant and in a preselected ratio. This is accomplished byowing the two streams continuously through the respective chambers of arefractometer cell to a common vent conduit.

Accordingly, it is an object of this invention to provide a differentialrefractometer which is particularly adapted to analyze gaseous streams.

Another object is to provide a system for maintaining ICC the pressuresof two gaseous streams in a predetermined ratio.

Another object is to provide a method for maintaining the pressures oftwo gaseous streams in a predetermined ratio.

Other objects, advantages and features of the invention should becomeapparent from the following detailed description which is taken inconjunction with the accompanying drawing in which:

FIGURE 1 is a schematic representation of apparatus which is provided tocrack hydrocarbons and separate the efliuent vapors.

FIGURE 2 is a schematic drawing of a differential refractometer, havingthe pressure control system of this invention incorporated therein,which can be employed to control the operation of the fractionationcolumn of FIGURE 1.

FIGURE 3 is a graphical representation of the concentration and index ofrefraction of materials in the lower region of the fractionation columnof FIGURE 1.

FIGURE 4 is a graphical representation of the concentration of methaneas a function of the refractive index of the fluid mixture in the lowerportion of the fractionation column of FIGURE 1.

Referring now to the drawing in detail and to FIG- URE 1 in particular,a hydrocarbon feed stream to be cracked is supplied by a conduit 10which communicates through a vaporizer 11 with the inlet of a crackingfurnace 12. A diluent, such as steam, is added to conduit 10 by means ofa conduit 13. Furnace 12 has an elongated pipe 14 in the upper portionthereof which communicates with a second elongated pipe 15 in the lowerportion. Heat is supplied to these pipes by a plurality of burners whichare mounted in the side walls of the furnace. Pipe 1S communicates withan outlet conduit 16. Water is added to conduit 16 by means of a conduit17 in order to quench the furnace efuent. Conduit 16 communicates withthe inlet of a quench drum 56. A cool oil is sprayed into drum 56 bymeans of an inlet conduit 18 to cool further the furnace effluent. Theheated oil is removed from the bottom of drum 56 through an outletconduit 19, cooled, and subsequently recycled to drum 56. Vapors fromdrum 56 are directed through a conduit 21 and a cooler 22 to a knock-outdrum 23. Any condensed material is removed from drum 23 through aconduit 24. Vapors are removed from drum 23 through a conduit 25 whichcommunicates with the inlet of a compressor 26. The outlet of compressor26 is connected by means of a conduit 27 to the inlet of an absorber 29.

A lean oil is supplied to the upper region of absorber 29 by means of aconduit 30. AThe rich oil is removed from absorber 29 through a conduit31. Absorber 29 normally is operated so that substantially all of the C4and heavier hydrocarbons are absorbed by the oil and removed throughconduit 31. The efuent gases from absorber 29 are removed through aconduit 32 which has a drier 33 therein. Conduit 32 communicates at asecond end with the inlet of a fractionation column 35 which.

is employed to remove methane and lighter gases from the gaseouseffluent from absorber 29.

Column 35 is provided with a cooling coil 36 in the upper region and aheating coil 37 in the lower region. These coils provide the desiredtemperature differential to accomplish the separation of the iiuidmixture. Vapors are removed from the top of column 35 through a conduit38 which communicates through a condenser 39 With a reflux accumulator40. The condensed liquid is directed from accumulator 40 back to column35 through a reflux conduit 41. The vapor in accumulator 40 is removedthrough a conduit 43 which communicates with a liquid-vapor separator 44through heat exchangers 45 and 46 and a pressure reducing valve 47.Condensed vapors are returned from separator 44 to accumulator 40through a conduit 48 which passes through heat exchanger 45. Cooledvapors from separator 44 are removed through an outlet conduit 50 whichpasses through heat exchanger 46. A control valve 51 is provided inconduit 50 to permit the amount of eiiluent gas withdrawnV to beregulated.

kettle product is removed from column 35 through an outlet conduit 52. Aconduit 53 communicates between a'lower region of column 35 and theinlet of a refractometer 54 in order to provide a sample stream to therefractorneter. This sample is vented through a conduit 55.Refractometer 54 provides an output signal which is representative ofthe refractive index of the sample stream removed from column 35. Thissignal contols valve 51 in the manner described in detail hereinafter toregulate the operation of the column so that the desired uid separationis maintained.

AV suitable instrument which can be employed to measure the refractiveindex of the sample stream is illustrated in FIGURE 2. This instrumentcomprises a' source of light 60 which is mounted in a housing 61. Source60 can be an ordinary incandescent bulb which emits radiation in thevisible spectrum. A portion of the light emitted from source 60 passesthrough 'a first aperture 62 and a converging lens 63. A narrow beam ofthis light emerges from housing 61 through a second aperture 64. Thislight beam is directed through a refractometer cell assembly 66. Thepurpose of aperture 62 is to reduce the total radiation transmitted fromlight source 60 in order to minimize heating of the cell assembly. Thefilament of source 60 is located slightly beyond the focal point of lens63 which in turn is positioned in close proximity to `aperture 64. Y

Cell assembly 66 comprises first and second chambers 67 and 68 which areseparated by a plate 69 of radiation transparent material. Plate 69 isdisposed at an angleV other than 90 with respectto Vthe axis of the beamof radiation transmitted through a cell assembly. A converging lens 71comprises the inlet window of chamber 68 and a converging lens 72comprises the outlet window of chamber 67. The components thus fardescribed are arranged such that aperture 64 is at the effectiveprincipal focus of lens 71. This results in'a parallel beam of lightbeing transmitted through the cell assembly.

The light beam emerging from lens 72 enters a glass prism 75 which ispositioned such that its front surface is substantially perpendicular tothe light beam. The

` light is twice reflected Within prism 75 and emerges in a direction soas to pass through a rotatable glass block 76. The lightbeam thenstrikes the apex of a second prism 77. A radiation detecting unit whichcomprises photovoltaic cells 78 and 79 is positioned so that a lightbeam striking the apex of prism 77 is divided into two beams of equalmagnitude which impinge upon the twoV cells.

The output voltages of cells 78 and 79 arerconnectedrin opposition toone anotherto the input terminals of a converter 81. The differentialoutput signal from the two cells is thus converted into a correspondingalternating signal which is applied to the input terminals of an amplierS2. Converter 81 is energized from a source'of alternating current 84which is applied across the primary winding 85 of a transformer 86through a switch 87. The first secondary winding 8S of transformer 86 isconnected to converter 81 so that the frequency of the signal applied toamplifier 82 is the same as that of current source 84. The output signalof amplifier 82 is applied to the first winding 89 of a reversible twophase induction motor 90. A second secondary winding 92 of transformer86 is connected across the second motor winding 93 through a capacitor94. This results in the two signals applied to the motor windings beingeither 90 or 270 out of phase with one another depending upon thepolarity of the input signal applied to converter 81. Converter 81,amplifier 82 and motor 90 can be of the type described in The ElectronicControl Handbook, Batcher and Moulic, Caldwell-Clements, Inc., New York,1946, pages 298 to 300, for example.

The drive shaft of motor is connected through gears 95 and 96 to a drum97. A wire 98 is attached to a rocker arm 99 which is connected to thebase 100 upon which block 76 rotates. Wire 98extends from rocker arm 99about guide wheels 101, 102, and 103 and drum 97. Rotation of drum 97thus results in block 76 being rotated about its pivot point. The driveshaft of motor 90 is also connected to the contactor of a telemeteringpotentiometer 105. A voltage source 106 is connected across the endterminals of potentiometer 105. The'contactor and one end terminal ofpotentiometer are connected to the input terminals of a controller 107.Controller 107 can be a conventional instrument which provides aregulated output air pressure which is a function of an input electricalsignal. This output air pressure can be employed to actuate valve 51 ofFIGURE l.

The sample stream removed from column 35 of FIG- URE l is directedthrough conduit 53 which communicates with the inlet of a pressureregulator 110. The outlet of pressure regulator 110 is connected by aconduit 111 to the inlet of chamber 67. The outlet of chamber 67 isconnected by a conduit 112 to the inlet of a valve 113. The outlet ofvalve 113 is connected to vent conduit 55. A reference uid is suppliedby a conduit 115 which communicates with the inlet of a pressureregulator 116. The outlet of pressure regulator 116 is connected by aconduit 117, which has a valve 118 therein, to the inlet of chamber 68.The outlet of chamber 68 is connected by a conduit 120 to the inlet ofvalve 113.

The refractive index measurement is made by comparing the refractiveindex of the sample stream with the refractive index of a referenceiiuid. Both of these fluids are maintained in the gaseous state. It isdesirable that the sample gas be maintained at a relatively highpressure to provide greater sensitivity. A pressure of approximately 300poundsV per square inch can be employed to advantage, for example.Pressure regulator-110 thus maintains the sample stream circulatedthrough chamber 67 at -this pressure. The reference uid in chamber 68can be a portion of the final ethylene productfrom the separationsystem. This material is applied to valve 118 at a constant pressure ofapproximately 315 pounds per inch, for example, by pressure regulator116. This results in a constant pressure differential across valvet118.

Pressure regulator 116 and valve 118 thus function as a` ow regulator.If the flow through valve 113 is greater than the iiow through valve 118and chamber 68, there is a iiow through chamber 67. Flow control system110 and 113 must be set to flow a sample rate larger than throughchamber 68. The material in chamber 68 canremain stationary, however. g

If the refractive index of the iiuid in chamber 67 is the same as therefractive index ofthe fluid in chamber 68, the light beam emerging fromcell assembly 66 is parallel to the light beam which enters `the cellassembly. Any change in refractive index of the sample iluid results ina deviation of the direction of the emerging light beam. The servosystem operates so that the light beam impinges upon the apex of prism77. This results in equal output voltages VbeingY provided by cells 78and 79 so that the differential outputl voltage is zero. Motor 90 thusremains stationary. If the light beam should deviate in eitherdirection, one of the photovoltaic cells receives a greater amount ofradiation than the other. The results in an output signal which drivesmotor 90 to rotate drum duced into furnace 12 through conduit 10. Foreach one thousand mols of feed gas, 249 mols of water in the form ofsteam are added through conduit 13. The resulting mixture is introducedinto furance 12 at a temperature of approximately 150 F. and at apressure of approximately 85 pounds per square inch absolute (p.s.i.a.).The residence time of the mixture in pipe 14 is approximately 7 secondsand the residence time in pipe 15 is approximately 4 seconds. The gasesare heated in pipe 14 Ito a temperature of approximately 1000 F. and arefurther heated in pipe 15 to a temperature of approximately 1500e F. Theefiiuent gases leave furnace 12 at a temperature of approximately 1000F. and at a pressure of approximately 2O p.s.i.a. The etiiuent fromfurnace 12 has a composition approximately as follows:

Mols per 1000 Absorber 29 is operated so that nearly all of the C3 andlighter constituents are removed overhead and nearly all of the C4 andheavier constituents are absorbed by the oil. The composition of the uidmixture in the lower region of column 35 is approximately as follows:

Theoretical Trays (22 Total) Kettle 1 2 3 4 o 6 7 Methme 0. 42 1. 06 2.37 4. 94 9. 79 18. 1l 30. 40 44. 9 75 37 79. 54 80. 11 78.61 74. 82 67.97 57. 72 45. 6

Benzene 0. 14 0. 006

In FIGURE 3 the index of refraction of the iiuid mixture at theseseveral trays is plotted against tray number. The methane concentrationat the same trays is also plotted. It can 1oe seen that the two curveshave substantially the same shape. Thus, the refractive index of themixture is representative of the methane content in the lower region ofthe column. A measurement of refractive index of a sample withdrawn froma lower tray, actual tray two to six, for example, is thus indicative ofthe .methane concentration. FIGURE 4 shows the methane concentrationvaries almost linearly with the refractive .index of the mixture. If themeasured refractive index decreases, for example, the methaneconcentration is increased. Valve 51 is then opended further to permit agreater withdrawal of overhead gas. Conversely, valve 51 is closedfurther if the measured refractive index increases.

Interferences which might be expected are not objectionable because ofthe relatively large change in methane concentration per tray. Forexample, at 10 percent methane, a variation as much as percent in theethylene-ethane ratio is indicated as only a one percent change inmethane concentration. The heavier components are present in very smallquantities so that changes in concentration thereof are relativelyunimportant.

While the invention has been described in conjunction with presentpreferred embodiments, it obviously is not limited thereto.

What is claimed is:

l1. A method of operating two zones adapted to transmit light through aHuid in each of said zones, which comprises flowing a first iiuidthrough a iirst pressure regulation zone, regulating the pressure ofsaid rst fluid and discharging said first uid from said first reguationzone at a predeterminedpressure, then flowing said first uid into afirst light transmitting zone, flowing said fluid from said zone into acommon zone and from said common zone, regulating ilow of total iiuidfrom said common zone, owing a second uid through a second pressureregulation zone, regulating the pressure of said Second iiuid, anddischarging said second uid from said second pressure regulation zone ata predetermined pressure, then owing said second uid into a second lighttransmitting zone, flowing said second fluid from said second lighttransmitting zone into said common zone into admixture therein with saidfirst fluid, and regulating the flow of said second uid from said secondregulating zone into said second zone and the flow of the combinedfluids from said common zone so that there is effected a flow of uidthrough at least one of said iirst and said second light transmittingzones.

2. A method according to claim 1 wherein in one of the lighttransmitting zones, there is contained a fluid to be analyzed and in theother light transmitting zone, there is contained a reference fluid,light is transmitter through said liuids and variations in thetransmitted light are employed to regulate an operation resulting insaid fluid to be analyzed.

References Cited in the tile of this patent UNITED STATES PATENTS1,231,293 Peters June 26, 1917 2,270,304 Jacobsson Ian. 20, 19422,724,304 Crawford Nov. 22, 1955 2,787,281 Word Apr. 2, 1957 2,868,216Robertson Jan. 13, 1959 2,885,922 Miller May 12, 1959

1. A METHOD OF OPERATING TWO ZONES ADAPTED TO TRANSMIT LIGHT THROUGH AFLUID IN EACH OF SAID ZONES, WHICH COMPRISES FLOWING A FIRST FLUIDTHROUGH A FIRST PRESSURE REGULATION ZONE, REGULATING THE PRESSURE OFSAID FIRST FLUID AND DISCHARGING SAID FIRST FLUID FROM SAID FIRSTREGUATION ZONE AT A PREDETERMINED PRESSURE, THEN FLOWING SAID FIRSTFLUID INTO A FIRST LIGHT TRANSMITTING ZONE, FLOWING SAID FLUID FROM SAIDZONE INTO A COMMON ZONE ND FROM SAID COMMON ZONE, REGULATING FLOW OFTOTAL FLUID FROM SAID COMMON ZONE, FLOWING A SECOND FLUID THROUGH ASECOND PRESSURE REGULATION ZONE, REGULATING THE PRESSURE OF SAID SECONDFLUID, AND DISCHARGING SAID SECOND FLUID FROM SAID